1. Field of the Invention
The present invention relates to a method for adjusting an x-ray device. More specifically, said x-ray device is of the type employed in the medical field and serves to perform radiographic and/or fluoroscopic examinations with or without the aid of luminance amplifiers.
2. Description of the Prior Art
An x-ray device is made up of an x-ray emitter, a patient support panel, and a device for measuring the irradiation produced by the x-ray tube after passage of radiation through a body positioned on the panel for examination. The measuring device can be associated with a television-type camera or a radiation-sensitive film. Handling of x-ray tubes is a difficult operation which involves electronic-electromagnetic conversion. Electrons emitted by a cathode impinge upon an anode at high velocity. Under the action of electron impacts, the anode emits the requisite x-radiation. However, the conversion ratio does not have the value of 1:1. Part of the energy of the incident electrons is converted to low-frequency electromagnetic radiation or in other words to heat. This results in heating of the anode which consequently has to be used with care.
Up to the present time, different expedients have been contemplated with a view to solving this problem. In one solution, the x-ray tubes are provided with cooling devices. In another solution, the anode of x-ray tubes is a rotating anode, with the result that the surface exposed to electron radiation is continuously renewed and, on an average, heats up to a lesser degree. In spite of these solutions, when the load imposed on the x-ray tube is too high, safety cutoff devices have to be contemplated. Thus a system of characteristic curves measured for each type of x-ray tube serves to establish in respect of given values of anode temperature and power the maximum period of time during which the x-ray tube can be employed in continuous operation before its anode attains an ultimate temperature value or temperature limit. This value cannot be exceeded without resulting in irreparable damage to the x-ray tube. The safety cutoff operation consists in continuously measuring the anode temperature and cutting-off the supply of current to the x-ray tube when the ultimate temperature is attained.
A set of procedures or methods of utilization of x-ray devices has been deduced from the above-mentioned safety cutoff operation. The methods are all based on the same principle and consist in computing the permissible operating time of the x-ray tube, on the one hand on the basis of the high-voltage power supply of the x-ray tube and the dose to be received by a radiation-sensitive receiver and on the other hand on the basis of the starting temperature of the anode. Either this utilization time is shorter than actual requirements and any experimentation is precluded since it cannot be carried on to completion or else the utilization time exceeds requirements and this results in a waste of time in the majority of cases. The anode temperature is a calculated datum which is specific to the x-ray tube at the moment of utilization. The high voltage, dose rate or sensitivity of a film are data relating to images to be produced. The high voltage of the x-ray tube governs the x-radiation spectrum or in other words the penetrating power of x-rays and is determined experimentally as a function of the zones to be imaged in a human body. The radiation dose or sensitivity of the film is a direct consequence of this high voltage and of customary practices. It corresponds to the product of the heating current of the cathode of the x-ray tube and the time-duration of cathode emission. This time-duration is of course the same as the period of utilization of the x-ray tube (utilization time). In the final analysis, the aforesaid product as expressed in milliampere-seconds (mA.s) is directly proportional to the film exposure. The film must have received a sufficient radiation dose to consider that it has been sufficiently exposed. The product of high voltage in kilovolts (Kv) and milliampere-seconds (mA.s) gives the radioelectric energy dispensed by the x-ray tube or in other words the resulting quantity of heat generated at the anode, to within the nearest conversion ratio.
In one method, for example, two radiographic data, namely kilovolts and milliampere-seconds, are set up separately by an operator. The utilization time is computed with reference to an x-ray tube load graph. The weakness of this method lies in the fact that, if several successive exposures are made, the utilization times increase in length as exposures are continued. Between each exposure, the anode temperature has in fact progressively risen and the margin of variation between the operating temperature and the ultimate temperature is diminishing. In one expedient adopted in sophisticated generators with a view to guarding against this drawback, an operator enters a coefficient k which is smaller than 1 and enables a microprocessor to compute the exposure time with a characteristic load curve which is homothetic with the real load curve within a factor k. In other words, the time-durations are not optimized and are longer by 1/k. However, when it is required to examine a human body in which there is some movement of certain parts of the body, it is necessary to choose the shortest possible time intervals in order to avoid motional blur. The conclusion is that, despite its operational reliability, this method is not the most satisfactory.
The aim of the present invention is to overcome the disadvantages mentioned in the foregoing while also simplifying the work required of the operator. In actual fact, an operator is not interested either in the anode temperature or in the factor of merit k to be attributed to an experiment. In the invention, it is only necessary to indicate the number of exposures N during which the x-ray tube is intended to operate. A microprocessor then computes the exposure times with a view to limiting them to the lowest possible values.